1. Field of the Invention
This invention relates to a structure of semiconductor device that utilizes a conductive layer formed near a hetero-junction interface of a compound semiconductor and a method of manufacturing the same.
2. Description of the Related Art
High electron mobility transistors (HEMTs) using a compound semiconductor, GaN for example, are used as high power and high frequency devices. FIG. 5 schematically shows a cross-sectional view of a known HEMT device 90. Referring to FIG. 5, a channel layer 92, an electron supply layer 93 and a cap layer 94 are sequentially formed on a substrate 91 by epitaxial growth. For example, the channel layer 92 is formed by semi-insulating (non-doped) GaN, the electronic supply layer 93 is formed by semi-insulating (non-doped) AlGaN (to be more accurate, semi-insulating (non-doped) AlxGa1-xN, where x=around 0.20) and the cap layer 94 is formed by n-type GaN. In this structure, a two-dimensional electron gas layer 95 (indicated by a broken line in FIG. 5) that becomes an conductive layer is formed near and in parallel with the interface (hetero interface) between the channel layer 92 and the electron supply layer 93 at the side of the channel layer 92. An electron current flows between a source electrode 96 and a drain electrode 97 due to the two-dimensional electron gas layer 95 and on/off of the channel formed by two-dimensional electron gas is controlled for switching operations by the voltage applied to the gate electrode 98 that becomes to operate as Schottky electrode. High-speed switching operations are possible because the speed (mobility) of electrons in the two-dimensional electron gas is very high. Since GaN has a wide band gap compared to GaAs or the like, the HEMT device 90 shows a high breakdown voltage and is adapted to high output operations.
In order to make the on-resistance of the HEMT device 90 low, it is necessary to reduce the resistance between the two-dimensional electron gas layer 95 and the source electrode 96 and between the two-dimensional electron gas layer 95 and the drain electrode 97, the source electrode 96 and drain electrode 97 being held in ohmic contact with the two-dimensional electron gas layer 95. This purpose can be effectively achieved by filling the electrode materials of these electrodes 96 and 97 in the respective recess structures obtained by locally etching the cap layer 94, the electron supply layer 93 and the channel layer 92 at places for receiving the source electrode 96 and the drain electrode 97 so as to make the source electrode 96 and the drain electrode 92 directly contact the two-dimensional electron gas layer 95 as shown in FIG. 5.
A technique of reducing the on-resistance by devising the shape of the source electrode 96 and that of the drain electrode 97 (or the etched shape at their places) in a particular way is described in Patent Document 1.
The on-resistance can be reduced to provide a high performance HEMT device by etching the cap layer 94, the electron supply layer 93 and the channel layer 92, using the shape and the depths as described in Patent Document 1, to produce recess structures and filling the electrode materials in the recess structures to obtain the source electrode 96 and the drain electrode 97.